A fuel cell is a system for producing electric power. In a fuel cell, chemical energy is directly converted into electric energy by using an electro-chemical reaction between oxygen and the hydrogen contained in a hydrocarbon-group material such as methanol, ethanol, or natural gas. Particularly, the fuel cell system is advantageous in that both the electric power generated through the electrochemical reaction between oxygen and hydrogen without any combustion process and the heat generated as a by-product thereof can be utilized at the same time.
Fuel cells can be classified depending on the type of electrolyte used and their corresponding operating temperatures. For example, a phosphate fuel cell has an operating temperature range of 150 to 200° C., a molten carbonate fuel cell has a higher operating temperature range of 600 to 700° C., a solid oxide fuel cell has a higher operating temperature range of over 1000° C., and a polymer electrolyte membrane fuel cell (PEMFC), and an alkali fuel cell have lower operating temperature ranges of below 100° C. or at room temperature. These different types of fuel cells work using the same basic principles, but differ from one another in their kinds of fuel, operating temperature, catalyst, and electrolyte.
The recently developed polymer electrolyte membrane fuel cell (PEMFC) has excellent output characteristics, a low operating temperature, and fast starting and response characteristics compared to other fuel cells. The PEMFC can be widely applied to mobile power sources used for vehicles, distributed power sources used for homes and buildings, small power sources used for electronic appliances, and the like.
PEMFC fuel cell systems comprise a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a main body of the fuel cell. The fuel pump supplies fuel from the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and then supplies the hydrogen gas to the stack. Accordingly, the PEMFC fuel cell system supplies the fuel from the fuel tank to the reformer through operation of the fuel pump and reforms the fuel with the reformer to generate hydrogen gas. Then, the stack generates electric energy through an electro-chemical reaction between the hydrogen gas and oxygen.
In contrast, a direct methanol fuel cell (DMFC) can generate electric power by directly supplying liquid fuel containing hydrogen to the stack, and does not require a reformer as is required for a PEMFC.
In the above-noted fuel cell system, the stack which substantially generates electric energy, is structured including a few to a few tens of unit cells, each comprising a membrane-electrode assembly (MEA), with separators (consisting of bipolar plates) provided on both sides thereof. In the MEA, an anode and a cathode are provided opposing one another with an electrolyte layer interposed therebetween. The separator functions as a pathway for providing hydrogen gas and oxygen gas, which are required for a fuel cell reaction, as well as a conductor for connecting the anode and the cathode of each MEA in series. Accordingly, through the separators, the hydrogen gas is supplied to the anode and the oxygen is supplied to the cathode. During this process, an oxidation reaction of the hydrogen gas occurs in the anode, and a reduction reaction of the oxygen occurs in the cathode, so that electric energy, heat, and water can be generated by electron movement occurring at the same time.
The aforementioned reformer converts water and liquid fuel containing hydrogen into the reformed gas rich in hydrogen gas required to generate electric energy in the stack through a catalytic reformation reaction. Also, the reformer reduces harmful materials such as carbon monoxide, which decreases a fuel cell's lifetime, by purifying the reformed gas. For these purposes, the reformer comprises a reformation unit for reforming the fuel to produce hydrogen gas, and a carbon monoxide elimination unit for reducing the amount of carbon monoxide in the reformed gas. The reformation unit converts the fuel into reformed gas rich in hydrogen through a catalytic reaction such as a moisture reformation, a partial oxidation, and a natural reaction. The carbon monoxide elimination unit reduces or eliminates carbon monoxide from the reformed gas through various methods including catalytic reactions such as by a hydrogen gas conversion, by selective oxidation, or by using separators.
In a conventional fuel cell system, the reformation unit is provided with a catalyst layer for reforming mixed fuel containing liquid fuel and water in a reactor. The reformation unit is heated and generates reformed gas rich in hydrogen from the mixed fuel through a catalytic reformation reaction in the catalyst layer.
However, the reformer of the conventional fuel cell system requires a separate process for forming the catalyst layer in the reactor, thereby increasing manufacturing costs. As a result, the cost of the entire system is also increased. In addition, when a metallic reactor is adopted, the reformation catalytic layer is formed on the reactor in such a way that an oxidation film is provided on the surface of the reactor, and then a catalyst solution is doped thereon. This complicates the manufacturing processes and causes the reformation catalytic layer to be easily exfoliated from the surface of the reactor.